Mitsubishi Mini-split fried twice due to a power quality problem of some sort.

Gasquet said:
If that were true, then why does it have a <3% specification on voltage imbalance? The compressor may not care, but the low voltage boards do. There is some digital communication on the power lines implemented, so this is where I think the weakness comes in. Go back to the link above and read the other thread. It's pretty relevant.
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What is your reference for voltage imbalance if the applied voltage is supposed to be 240 +-? You are getting more now. Where does it ask for Line to Ground voltage?

A buck/boost will make your L-G worse. An isolation transformer will help your electrician get his kids thru college, but have at it.

What are you using for backup heat? Gas?



Does this unit have gas heat for backup?
 
Gasquet said:
Focusing the discussion. Would you use an isolation transformer, or just a buck boost transformer (assuming that this heat pump does indeed have a reference to neutral in the low voltage circuitry)? Also, assume that for now, we will not find the problem with the neutral. Normally there is heat associated with an issue on the neutral. Can't find heat.
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If you are going to go to the trouble of adding some kind of transformer might as well put in an isolation xfmr. You can make a neutral on the secondary side of the transformer and you can buy a transformer with some taps to allow you to tweak the voltage if needed.

I don't think the transformer is going to do much good though if you have power that is failing regularly. Mostly it will just pass thru from the input to the output without attenuating any surges. It might remove some higher frequency noise but that does not seem to be your issue.

You might want to consider a power filter. Something like this maybe. Keep in mind these kind of things require some application experience to properly select. It is not always a simple thing to decide based on the sales blurbs.

 
Gasquet said:
If that were true, then why does it have a <3% specification on voltage imbalance? The compressor may not care, but the low voltage boards do. There is some digital communication on the power lines implemented, so this is where I think the weakness comes in. Go back to the link above and read the other thread. It's pretty relevant.
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Do the low voltage boards have any external ground reference?

Do the low voltage boards have any connection to the neutral?

Ask yourself if a sensor on the low voltage board could measure the voltage imbalance. If the board cannot measure the imbalance, then the imbalance cannot affect the board.

Now imagine that the board generates a midpoint voltage relative to the supply legs. On a balanced 120/240V system, this midpoint voltage will be near ground potential. If the supply is not balanced, the midpoint will be offset from ground voltage.

If the low voltage boards were somehow sensitive to the midpoint to ground voltage, then you might say 'unbalanced supply increases midpoint to ground voltage, and excesd midpoint to ground voltage causes failures.' But this is flawed, because a perfectly balanced 120/208V system would also have an elevated midpoint to ground voltage.

This is why I believe the leg balance requirement is a red herring. Leg balance is critical in a 3 phase system where the motors are directly powered from the 3 phase supply. I believe that the balance value is boilerplate from a 3 phase spec.

IMHO you should get Mitsubishi to tell you why leg voltage balance matters.
 
If your only goal is to reduce the voltage at the heat pump, the buck transformer is as good as a fully-rated two-winding isolation transformer.

A two-winding transformer will provide some buffering impedance between the heat pump and the rest of your system. Without actually determining what the problem is, it's hard to know if this will be beneficial. More equipment cost, more protection required and more energy costs.
I'd also want to look more closely at the failure mode of the circuit boards. Has the control power voltage at the board been measured? A diagnosis of a "fried" control board isn't really too specific.

Good luck.
 
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